This invention pertains generally to television receivers. In particular it pertains to means for altering the overall frequency response of a television IF (intermediate frequency) amplifier system whose frequency response is determined in part by nonvariable tuning elements such as acoustic filters commonly called SWIFs (surface wave integratable filters).
Basically, an acoustic surface wave device comprises a piezoelectric medium propagative of acoustic surface waves having an input transducer coupled to the propagating surface of the medium for launching acoustic surface waves in response to an applied electrical signal. The waves propagate across the surface to one or more output or receiving transducers also coupled to the medium. An electrical output is generated by the interaction of the output transducer(s) and the propagating acoustic surface waves. By appropriate selection of the medium material and the design of the transducers, a wide variety of different frequency selective characteristics may be obtained. In addition, one or more SWIFs may be connected in a signal transmission path to provide additional selectivity.
These acoustic wave devices may be fabricated by integrated circuit technology such that the entire frequency selection of an IF amplifier can be realized on a small piezoelectric substrate. Because of their small size and method of fabrication, SWIFs lend themselves readily to use in the compact low signal level environment typical of solid state systems, particularly those using integrated circuits.
The characteristics of SWIFs which make their use in IF amplifiers desirable also impose certain limitations on their use. While their ability to be mass-produced in accordance with integrated circuit technology and the fact that their fixed geometry provides a fixed predetermined frequency response offers obvious advantages, it also imposes a limitation on their application in television systems due to an inability to vary their frequency response.
It has been found by practitioners in the television art that it is desirable under some conditions to alter the frequency response of the IF amplifier. For example, it is often advantageous to vary IF system response as a function of the strength of signal received. Normally (that is when a strong signal is received) the picture IF and sound IF carriers are positioned on opposite slopes of the IF bandpass curve. Typically the picture IF carrier is approximately 6 decibels (dB) below the peak response. The strong signal level of the sound IF carrier is approximately 10 to 20dB below peak response. However, when the strength of signal received decreases substantially, the well-known action of the AGC (automatic gain control) system responds by increasing system gain which maintains the amplitude of the IF signal at its preselected level. Unfortunately this increased gain also increases the noise content of the signal which manifests itself as "snow" or graininess in the displayed image. For these reasons, it has been found to be desirable to adjust the frequency response of the IF amplifier such that the picture carrier is emphasized, that is, positioned at the peak of the IF response curve. This procedure effectively enhances the gain of the IF amplifier at the frequency of the picture carrier. Because the gain of the IF amplifier is increased primarily for signal components at or near the picture IF carrier frequency, the noise components of the signal (generally signals away from the picture IF carrier) are reduced.
Methods of such picture carrier enhancement are set forth in the U.S. Pat. No. 3,872,387 issued to Frank G. Banach and assigned to the assignee of the present invention which shows a variable bandwidth means having a passband substantially centered at the picture IF carrier frequency. The bandwidth means are responsive to the control voltage of the AGC system. Under strond signal conditions the bandwidth means exhibit a broad passband and have little effect upon the overall IF system frequency characteristic. Under weak signal conditions the bandwidth means exhibit a narrow passband enhancing the picture IF carrier relative to the remainder of the IF signal components.
Such systems perform well and achieve substantial performance improvements. They are particularly effective when used in receivers having separate detectors for the picture and sound IF carriers. Some difficulties are found however when a common detector is used to recover both the picture and sound information such with the synchronous demodulation systems which are now finding increased use in the television art. In such detectors, there is an acceptable range of sound IF carrier levels which can be properly detected. Excessive sound IF carrier results in undesired production of "cross product" interference signals. The most apparent of which is the well-known chrominance-sound beat which results in a 920KHz interference signal in the displayed image. Insufficient sound IF carrier on the other hand leads to objectionably "noisy" audio reproduction or even total loss of audio output. With this limitation on strong signal sound IF carrier level, a difficulty arises in variable bandwidth systems because the change of IF system bandwidth under weak signal conditions reduces not only the noise components, but also the amplitude of the sound IF carrier available for detection. Until now, this effect has resulted in a limitation upon the degree of bandwidth variation used in fixed-tuned IF systems.
Accordingly, it is a broad object of the present invention to provide an improved fixed-tuned IF amplifier system for use in a television receiver. It is a more particular objective to provide bandwidth variation of a fixed-tuned IF amplifier without objectionable loss of sound information components.